Viega 20783 ProPress reducer Smart Connect technology Copper, 4'' x 3''

The 20783 is a FTG x P reducer. The 4 inch end is a fitting (street) end sized to slip into the socket of another 4 inch copper fitting, and the 3 inch end is a press socket that accepts 3 inch copper tube. So it goes fitting-to-tube, not tube-to-tube. If your 4 inch side is plain copper tube rather than a fitting socket, you need a P x P reducer instead.

No flame. ProPress joints are made cold with a press tool and the correct 3 inch jaw or ring for the press end; the 4 inch fitting end presses into whatever 4 inch ProPress fitting socket it lands in. Nothing about the 20783 calls for solder, flux, or a torch, which is why it gets used on occupied jobs where hot work permits are a problem.

That is exactly the transition it makes, provided the 4 inch side terminates in a ProPress fitting socket the street end can enter. The 3 inch press socket then takes 3 inch copper tube. Standardizing the step-down on one press fitting keeps the riser flameless end to end, which is the usual reason it gets specified on occupied buildings.

Copper. The 20783 ProPress reducer is a copper-body fitting, which is what you want in a copper potable or hydronic line so you are not mixing materials at the joint.

Smart Connect is the feature in the press socket that lets an unpressed connection leak visibly during pressure testing, so a joint you missed shows itself instead of hiding behind a wall. On a 4 x 3 transition that gets pressed once with a large jaw, that is the check you rely on before closing the ceiling.